On the addon page for FF Relay, scroll all the way down and click Privacy. When the page comes up, change "websites" at the end of the address to "firefox-relay", hit Enter. Scroll all the way down to "Information We Share".

I, personally, don't feel comfortable with Amazon being given access to that information. Attempts to remove personally-identifiable information have been bypasses many times before and I have no doubt that whatever safeguards Mozilla has in place will eventually be worked-around.

It's a nice concept but ruined by involving a company known for their pervasive tracking.